Collaborative Research: Dissolved organic phosphorus controls on marine nitrogen fixation and export production

浮游植物在地球的碳和氮元素循环中起着重要作用。除了阳光，生活在海洋表层的浮游植物还需要氮和磷元素来生长。在冬季的几个月里，当水柱的垂直稳定性被破坏时，这些营养物质中的大部分以无机形式由深水向地表混合而来。然而，北纬45度至45度的大部分低纬度海洋，由于上层水柱的强烈热分层（垂直稳定性），受到阳光照射的地表水的营养输入有限。因此，热带和亚热带浮游植物想出了获取氮和磷的替代方法。海洋固氮是一种特殊的微生物利用大气中丰富的氮气将元素氮转化为生物可利用的氨的过程。这些固氮浮游植物和许多其他植物也利用低纬度海洋中有机形式的磷，那里通常缺乏无机营养物。该项目将大大增加溶解有机氮和溶解有机磷浓度测量的数量，特别是目前采样不足的太平洋和印度洋。在数值模型中，海洋表面有机营养物浓度的变化将被用来推断浮游植物利用有机营养物的速率。具体而言，将通过结合数据和模型输出来量化低纬度海洋中溶解有机磷的生物吸收对海洋固氮和光合作用过程的促进作用。该项目将培养一名研究生和几名本科生在实验室化学分析技术和海洋生物和化学过程的数值模拟。新的科学知识将通过社交媒体活动与公众分享，并将为下一代全球气候模型的开发提供信息。海洋生物地球化学模拟界已经确定，全球海洋300米以上的溶解有机氮（DON），特别是溶解有机磷（DOP）浓度测量的缺乏，是我们准确模拟亚热带出口生产和N2固定率的能力的关键空白。拟议的工作将显著增加海洋DON和DOP浓度测量的全球数据覆盖范围，特别是在印度洋、热带南太平洋西部、中部和东部、阿拉斯加湾、东部亚热带和亚极地南太平洋、南大洋、亚热带北大西洋和热带南大西洋盆地等取样不足的海洋区域。这些新的测量结果将被吸收到最先进的生物地球化学模型中，以限制DOP和DON的相对循环速率，并量化DOP优先消耗作为支持低纬度海洋出口生产和N2固定的P源的作用。模型输出将求解以DON和DOP、自养DOP吸收率以及DON和DOP再矿化率积累的新产量的区域分解部分。该模式的输出还将包括第一个区域可变速率估计，即在太平洋和印度洋观测的约束下，对维持出口生产和固氮的发光区DOP消耗进行估算。因此，新的浓度测量和诊断模型将使我们能够评估DOP消耗和回收的区域变化的定量作用，这些变化支持低纬度海洋的出口生产和N2固定。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Phytoplankton play an important role in the Earth's elemental cycles of carbon and nitrogen. In addition to sunlight, phytoplankton living in the surface waters of the oceans require the elements nitrogen and phosphorus for growth. Much of these nutrients are supplied in their inorganic forms from mixing of deep waters towards the surface during the winter months when vertical stability of the water column breaks down. However much of the low latitude oceans, 45degS45degN, suffer from limited nutrient input to sunlit surface waters due to strong thermal stratification (vertical stability) of the upper water column. As a consequence, tropical and subtropical phytoplankton have devised alternative ways of acquiring nitrogen and phosphorus. Marine nitrogen fixation is a process by which specialized microbes utilize the abundant nitrogen gas from the atmosphere to convert elemental nitrogen into the bioavailable form ammonia. These nitrogen fixing phytoplankton and many others also use organic forms of phosphorus in the low latitude ocean where inorganic nutrients are often scarce. This project will significantly increase the number of dissolved organic nitrogen and dissolved organic phosphorus concentration measurements, especially from the currently under-sampled Pacific and Indian Oceans. Changes in the concentration of organic nutrients across the surface ocean will be used to infer rates of organic nutrient use by phytoplankton in numerical models. Specifically, the role for the biological uptake of dissolved organic phosphorus to stimulate the processes of marine nitrogen fixation and photosynthesis in the low latitude ocean will be quantified from the combined data and model output. The project will train one graduate student and several undergraduate students in both laboratory chemical analysis techniques and numerical simulation of ocean biological and chemical processes. New scientific knowledge will be shared with the public via a social media campaign and will inform the development of the next generation of global climate models. The marine biogeochemical modeling community has identified the lack of dissolved organic nitrogen (DON) and especially dissolved organic phosphorus (DOP) concentration measurements from the upper 300 m of the global ocean as crucial gaps in our ability to accurately model export production and N2 fixation rates in the subtropics. The proposed work will significantly increase global data coverage of marine DON and DOP concentration measurements, in particular from under-sampled ocean regions in the Indian Ocean, western, central, and eastern tropical South Pacific, Gulf of Alaska, eastern subtropical and subpolar South Pacific, Southern Ocean, subtropical North Atlantic, and tropical South Atlantic Ocean basins. These new measurements will be assimilated in state-of-the-art biogeochemical models to constrain the relative cycling rates of DOP and DON and to quantify the role of preferential DOP consumption as a P source supporting export production and N2 fixation in the low latitude ocean. Model output will solve for the regionally-resolved fraction of new production that accumulates as DON and DOP, autotrophic DOP uptake rates, as well as the remineralization rates for DON and DOP. The model output will also include the first regionally variable rate estimates of euphotic zone DOP consumption sustaining export production and N2 fixation to be constrained by observations from the Pacific and Indian Oceans. Thus, the new concentration measurements and diagnostic modeling will allow us to evaluate the quantitative role for regional variability in DOP consumption and recycling that supports export production and N2 fixation in the low latitude ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.